It is known to detect the contact of an aspirator tip by the liquid into which the tip is lowered for purposes of aspiration. A highly preferred method is by expelling pressurized air of constant pressure out of the tip orifice, the pressure inside the tip being detected by a pressure detector or transducer. When the latter detects an increase in pressure due to liquid encounter, the source of pressurized air is shut off from the tip, and the tip can then be evacuated in a conventional manner by a pump, to aspirate the liquid prior to dispensing it elsewhere. A representative patent depicting this arrangement is U.S. Pat. No. 5,013,529, especially FIG. 3 thereof.
With such a mechanism, there is a disadvantage: the shut-off valve disconnecting the tip from the pressurized source is unable to always prevent such a build-up of pressure as will cause one or more bubbles of variable size to be expelled from the tip into the liquid covering the tip orifice. Such bubbles are undesirable, because unless they disperse completely, they can be aspirated back into the tip in place of liquid volume, thus destroying the accuracy of the volume of liquid that is expected. More specifically: the system uses a pressure transducer to detect a spike of increased pressure representing the wall of liquid now blocking the tip orifice. The spike of pressure is essential for the transducer to work. Unfortunately, it is this spike which then creates an air bubble at the tip orifice. The bubble at best sits at the tip orifice, only to be aspirated in during the aspiration cycle of the tip. In such a case, the air bubble volume becomes part of, and affects, only the residual volume of the tip and is never "dispensed". As such, then, such a bubble creates a second order effect by effectively changing the dead air volume in the air displacement system. For example, if the bubble volume represents 10% of an aspirated aliquot of liquid, its effect on what is subsequently dispensed is perhaps only about 2%.
The situation is worse, however, if the bubble disassociates from the tip, which happens on occasion. Then, the bubble could be aspirated back into the tip as part of the "slug" of volume that is to be dispensed. The bubble will migrate into the residual volume. However, timing is such that this migration is not always completed before the dispense operation (depending on the fluid properties of the sample and the initial location of the bubble). In that case, the bubble creates a first order error--a bubble volume representing 10% of the dispensed aliquot volume ends up as a 10% error in that dispensed aliquot.
In either case, either error is not acceptable for precise metering modes.
The problem cannot be solved merely by reducing the total pressure P.sub.T of the compressed air source of air pressure that is being expelled, with the intent of having less pressure build-up in the tip at the time of disconnect of the source of air pressure. The spike of pressure .DELTA.P.sub.1 is still needed within the tip to trigger the pressure detector to cause the disconnect. Thus, even if the P.sub.T of the source of air is reduced by half, so that the spike pressure .DELTA.P.sub.2 is, for example, only one half of the spike .DELTA.P.sub.1 that occurs for disconnect in the apparatus of said '529 patent, that is still enough pressure to cause a bubble to be ejected from the tip into the liquid. Because no such bubbles can be tolerated, even simply reducing by half the P.sub.T of the pressurized air source, is not an adequate solution. Furthermore, any attempt to greatly reduce the pressure spike that triggers the detector, to reduce the bubble size, quickly reaches a limit imposed by the signal-to-noise ratio required for a detectable spike.